Description

Established imaging and diffraction techniques for measuring structure of nanomaterials and soft matter do not show both good contrast and high resolution, and they can cause significant material damage. This is particularly the case for isolated nanostructures such as individual nanoparticles and ultrathin films. For example, identifying the crystallographic phase of an unknown nanoparticle of diameter < 5 nm is extremely difficult, even in the most powerful high-resolution TEMs commercially available today. The problem centers on the generation of electron scattering within small volumes. For structures in the size regime of 10 nm and smaller, electrons with energies of the order of 200 keV exhibit a mean free path for scattering that can easily approach an order of magnitude larger than the particle itself. Decreasing those energies to those typical of an SEM (~ 20 keV) decreases the mean free path to values commensurate with the particle size. As a result, more electrons will scatter, to provide the information-rich content needed to measure crystal phase, crystal orientation, defects, and internal order within nanostructures.

A fully analytical transmission-SEM, or STEM-in-SEM, would not only fuel more thorough characterization of nanoscale structures, enabling more precise process control and material reliability, but it would make available many powerful TEM-like capabilities to a large population of SEM users worldwide, in a diverse range of applications.

Major Accomplishments

Invented t-EBSD (aka transmission Kikuchi diffraction in the SEM), and demonstrated that the important Kikuchi scattering responsible for diffraction appears to occur within a few nanometers of the exit surface of the specimen.

Demonstrated that t-EBSD measurements can be made over a broad range of film thickness, ranging from < 5 nm to > 3 μm. This can be explained by considering effects of mass-thickness on electron penetration.

Developed a modular aperture system that significantly improves angular selectivity in the collection of electrons transmitted through a material, as compared to state of the art commercial detectors available today. Acceptance half-angles can be varied from zero to 1350 mrad, with spread controllable to single milliradians, depending on aperture fabrication methods.

Developed a programmable-aperture STEM detector capable of displaying a full diffraction pattern as well as synchronizing a portion of a diffraction pattern with the SEM scan – this results in the ability to generate images associated with any combination of scattering angles, including bright-field, annular bright-field, low-angle annular dark-field, medium-angle annular dark-field, and high-angle annular dark-field conditions. The detector is capable of defining any arbitrary subset of acceptance angles as the basis for imaging.